Rabbit 3000, User Manual

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218 Rabbit 3000 Microprocessor
16.7 Battery-Backed Clock Current Consumption
When using the suggested tiny logic oscillator, the oscillator and clock consume current as
shown in Figure 16-10 below. Normally a resistor is placed in the battery circuit to limit
the current to about 3 µA, which results in a voltage setpoint of about 1.7 V. When operat-
ing at 3.3 V in sleepy mode, the current of the oscillator and the real-time clock—about
12 µA—must be added.
Using the suggested tiny logic oscillator circuit, the external 32.768 kHz oscillator con-
sumes the following current in µA, where V is the operating voltage.
Iosc = 0.35×V
2
+ 0.31×V
Generally the oscillator will not start unless the voltage is about 1.4 V. However, the oscil-
lator will continue to run until the voltage drops to about 0.8 V. If the oscillator stops, the
current draw is very much lower than when it is running. Below about 1.4 V most of the
current draw is used to charge and discharge the capactive load.
The current consumed by the battery-backed portion of the Rabbit 3000, which is driven
by the 32.768 kHz oscillator, is given by
Irab = 0.91×V
2
- 1.04×V (V > 1.14 V)
where Irab is in µA. For V <1.14 V, the current is negligible.
Figure 16-10. Current Consumption—Real-Time Clock and 32 kHz Oscillator Circuit
0.00
2.00
4.00
6.00
8.00
10.00
12.00
1
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
3
3.2
Battery-Backup Voltage (V)
Curre
nt (µ
A)
Total Battery Backed
R3000 Real Time Clock
Tiny Logic 32 kHz Osc